1. Field of the Invention
The present invention generally relates to apparatus used to detect drug impairment, and more particularly to those systems with infrared illumination which monitor eye movement, eye lid blinking and pupil reaction to sources of radiation in the visible portion of the optical spectrum.
2. Prior Art
There are numerous devices and methods disclosed by the prior art which are used for analyzing physiological data in general, and eye movement in particular, as a diagnostic tool in the clinical investigation of organic disorders or drug impairment. It is understood the term "impairment" is intended to refer to the diminishing in strength, value, quantity or quality of the performance of activities of an individual. In recent years, a number of devices and methods have been disclosed which are specifically oriented to the detection, evaluation and analysis of disorders caused by ingestion of drugs and the resulting physiologic changes caused thereby. Throughout this application, reference is made to the effects of impairment by drugs. It is understood the term "drugs" includes not only medicinal substances and addictive narcotics, but also is to be understood to include any other ingested substance which affects the brain, as well as alcoholic beverages and the like. The techniques which are disclosed by the prior art are generally identified as electronystagmography (ENG), electro-oculographic (EOG), photoelectronystagmography (PENG) and Frenzel lenses.
There is a general consensus that eye movement may be employed as indicia for determining the sensory and motor aspects of the oculomotor system. This principle is made use of by stimulating the eye or eyes in some manner and evaluating the eye response to such a stimulus. Nystagmus is involuntary movement of the eyes. The nystagmus of the human eye will vary depending upon the organic disorder or the cause thereof. In measuring nystagmus, the devices disclosed by the prior art (ENG and EOG) use the inherent electrical potential between the cornea and the retina of the eye (the corneal-retinal potential) as a means for recording eye movement. This technique is based on the fact the eye acts as an electric dipole with the cornea being positively charged relative to the retina. The dipole axis is said to correspond to the visual axis. In the devices taught by the prior art, electrodes are placed at the skin surface adjacent to the eye orbit in an attempt to indirectly measure changes in electrical potential. The potential changes resulting from eye movement are recorded on various types of polygraph instruments. An ENG system uses a time constant and is generally used to record eye movement, whereas eye positions as well as eye movements are recorded using an EOG system.
Both the ENG and EOG systems exhibit inherent problems. Skin potential responses are in the same general frequency band as the eye movements, and some subjects produce signals having greater amplitudes than the corneal-retinal potential. The second problem is incident to the polarization of the skin electrodes which causes baseline shifts in the recorded corneal-retinal potential. This may inhibit or even prevent the detection and recording of resulting eye movements. In both cases, detection and evaluation of eye lid blinks is, at best, extremely difficult.
Some of the electronic problems associated with ENG and EOG recording methods are overcome with the system defined as PENG. This technique, using infrared sensitive photoelectric cells mounted on goggles, is able to detect the difference in reflected light off the sclera relative to the iris. The difference in reflectivity during eye movement is translated into an electrical signal which can be recorded on a polygraph instrument similar to that used with ENG or EOG methods. With this technique, the problem of electropolarization or skin potential responses is eliminated as no connection is made to the skin. However, eyelid tremor and blinking still cause artifacts which complicate the analysis of the polygraph traces. The major disadvantage of PENG is that testing must be done with the eyes open and in some cases, the infrared light used was of a shorter wave length and thus could be seen by the patient after a period of dark adaptation. The visible light source provides a target for optic fixation which may reduce or inhibit the nystagmus. The problem of resolving eye position and blink responses with PENG is the same as that encountered with electro-techniques.
A number of the devices and methods taught by the prior art utilize the accepted principle that drug ingestion may affect nystagmus. One of the methods taught by the prior art is set forth in the U.S. Pat. No. 4,576,184. The method taught in the '184 patent comprises the steps of placing electrodes of an ENG device on the face of the subject and thereby record the corneal-retinal potential and/or brainwaves caused by drug ingestion, recording the resting eye movement activity of the subject during a static positional test and then comparing the resulting waveform with those which are purportedly characteristic of ingestion of different drugs. The problems with this device are inherent in the manner in which the waveforms are detected. Rather than directly monitoring eye movement and pupil reaction to optic stimuli, the method employs the indirect measurement of the potential difference between the cornea and retina of the eye. As stated hereinabove, this method introduces factors which can mitigate the effectiveness of measurement and detection of drug impairment. In addition to other inadequacies, the device taught by the '184 patent cannot be used to detect and analyze eye lid blink activity.
Another device taught by the prior art is set forth in Applicant's U.S. Pat. No. 4,815,839. The device taught in the '839 patent employs an infrared camera to monitor the movement of the eye upon being bathed in fully dispersed infrared radiation. As with the present invention, the device taught in this patent precludes the impingement of ambient light to prevent optic fixation. The problems inherent in this device relate to the dispersement of the impinging infrared radiation and the limitations imposed by the manner in which the pupil and eye of the subject are monitored. Using a fully dispersed source of infrared radiation will degrade the ability of monitoring the pupil of the eye. A video camera sensitive to infrared radiation will detect a diffused reflection of energy across the entire surface of the cornea resulting in a reduced contrast between the pupil and the adjacent elements, i.e., the pupil will appear to be visually lighter than absolute black. Under such conditions, objects that do not reflect light such as eyelashes and eyelid makeup, may appear to be darker than the pupil of the subject. Since measurement of the pupil was not an object of the device taught in the patent, the problem could be ignored.
The present invention solves the problems inherent in those taught by the prior art by appropriately positioning point-sources of infrared radiation relative to the monitoring camera and thereby preclude detection of reflected energy in the pupil area. For example, in measuring physiologic data relative to detecting influences from drugs, the lateral and downward displacement of the infrared sensitive camera allows full monitoring of the pupil along its horizontal axis. This offsets the effect of cosine compression to the region of eye motion that is of minimal importance. The monitored data is directly converted into signal indicia responsive to pupil size and position.